This is a revised competing renewal application describing a proposal which focuses on hereditary hemochromatosis, one of the most common inherited diseases in humans. We plan to determine the prevalence of disease related morbidity in individuals homozygous for the hemochromatosis mutations, to isolate and characterize the hemochromatosis gene. The application describes two inter-related projects. Project 1 is a population based survey based on the ascertainment of a large number of clinically unselected hemochromatosis homozygotes. These individuals will be selected for study solely on the basis of HLA identity to a sibling who has been diagnosed as a homozygote. We will determine the frequency of clinical manifestations, iron loading without clinical manifestations, and the absence of iron loading in the study subjects. Our preliminary results on the first 86 clinically unselected homozygote siblings of known probands indicate that the majority of homozygotes develop iron overload, justifying the need for large-scale screening programs. The investigators have now collected 197 well characterized hemochromatosis pedigrees. DNA collected from pedigree members, along with duodenal mucosal biopsies obtained from a large number of homozygotes and normal controls affords a unique set of reagents to be used in Project 2. Project 2 will employ a positional cloning approach aimed at isolating and characterizing the hemochromatosis gene. The centromeric border of the region of interest has been defined through the study of rare recombinant events that were detected in our large pedigree collection. The telomeric border will be similarly defined and cDNA clones originating from the region will be identified, characterized and assigned a priority rating as hemochromatosis gene candidates. Assignment will be based on sequence information and tissue expression. We hypothesize that the regulation of iron uptake is highly conserved across mammalian species. Once the human gene is in hand we plan to clone the corresponding locus in the mouse and to create an animal model of hemochromatosis using homologous recombination. These studies will define the mechanism by which intestinal iron uptake is regulated.